1. Field of the Invention
This invention relates generally to optical amplifiers and more specifically to polarization diversity in optical amplifiers.
2. Discussion of the Related Art
In fiber-optic communication systems, propagating optical signals often arrive at network nodes with unknown polarizations. For example, polarizations of the arriving optical signals may vary unpredictably in time. The absence of a priori knowledge about the polarizations of the arriving optical signals makes it desirable to process such optical signals in a manner that is insensitive to polarization. For that reason, optical devices for processing optical signals are typically constructed to be polarization insensitive or independent; i.e., able to provide comparable performance regardless of the polarization of the input signal to the device.
Examples of devices that should be polarization independent are optical wavelength-converters and optical amplifiers (OAs).
If a component is highly polarization sensitive, then one technique to achieve polarization insensitive operation of the component is to use polarization diversity. To achieve polarization diversity in such devices (e.g., in optical wavelength-converters) it is known to split an arriving optical signal into two orthogonal polarization components and process the two polarization components in separate optical wavelength-conversion media. Ordinary optical wavelength-conversion media are polarization-sensitive. The optical wavelength-converters recombine the light produced in the separate ordinary optical wavelength-conversion media to produce an output optical signal. By splitting, separately wavelength-converting, and then recombining, such optical wavelength-converters can produce optical signals whose power at a converted-wavelength is independent of the polarization of the original arriving optical signal.
Using separate ordinary optical media to wavelength-convert the orthogonal polarization components of an arriving optical signal requires controls. In particular, environmental conditions such as temperature may affect wavelength-conversion in the ordinary optical media. Temporal variations in conditions of the separate optical wavelength-conversion media could destroy the polarization-diversity of the overall optical wavelength-conversion process. To avoid losing polarization-diversity, some optical wavelength-converters include devices that maintain their environmental conditions at constant levels. These environmental control devices are often costly and complex to operate.
Similar limitations would apply to an OA if the amplifying medium is polarization sensitive, such as with many semiconductor OAs.